RESULTS OF AN IRRIGATION EXPERIMENT ON STUART PECAN TREES IN EAST TEXAS IN 1956Pecan Research
A. O. ALBEN Crops Research Division Agricultural Research Service U. S. Department of Agriculture Interest in supplemental irrigation of many crops has been increasing in the South in recent years. Cotton, corn, pastures, and other crop plants have benefited by added water in the recent drought years. MATERIALS AND METHODS Water from a deep well and sprinkler irrigation equipment was available in 1956 so that it could be applied to a portion of a large east Texas pecan orchard. Production and quality records on the crop from the Stuart trees were obtained. The Stuart trees were 47 years old, growing on Norfolk fine sand. The experimental block consisted of thinned trees spaced 60x80 feet apart and unthinned trees spaced 40x60 feet. Irregularities of spacing occurred because of uneven thinning and missing trees. The orchard was planted to winter oats and vetch in late September 1955 and 250 pounds per acre of 10-20-10 fertilizer was applied at seeding time. The oat cover crop was grazed beginning in December 1955. The pecan orchard became so dry in March 1956 that the entire Stuart block later used for an irrigation experiment was given 2 inches*of water March 17 and 18; thus the check trees of the experiment had 2 inches of irrigation water. Two other rates of irrigation were used: 9 inches and 11. The pecan trees that received 9 inches of water were irrigated as follows: March 17 and 18, 2 inches; May 20 and 21, 2; July 18 and 19, 2; August 23 and 24, 112; August 30 and 31, 11. The trees that received 11 inches of water were irrigated the same except for application of 4 inches of water on July 18 and 19. Thinned and unthinned trees were given the same irrigation. Guard rows were established between the areas that received different rates of irrigation. For each treatment yield records were taken from 8 of the thinned and 10 of the unthinned trees matched as to size and thinning exposure. An excellent pistillate bloom occurred in the trees in 1956. The set of nuts was good on both thinned and unthinned trees. The trees had not been irrigated in 1955; therefore, their performance was conditioned by the water that came from rainfall. As the rainfall of 1955 was a ---------- *All water applied in acre inches. factor in setting the crop of nuts for 1956 and the rainfall of 1956 was a factor in retaining the nuts on the trees as well as influencing size and filling of the nuts, the rainfall for 1955 and that for the first 9 months of 1956, as well as the average for 44 years, are given in table I. The rainfall for 1955 and 1956 was recorded at the orchard. The average for 44 years was recorded at a Texas experiment station 16 miles from the orchard. Table I. Monthly Rainfall for 1955 and for January-September 1956, Experimental Orchard, and Average for 44 Years, 16 Miles from Orchard. Rainfall (inches) Month 1955 1956 44-Yr. Average January 2.34 3.65 3.50 February 4.31 3.35 3.40 March 4.66 .46 4.21 April 3.91 2.62 4.83 May 3.02 4.80 5.10 June .42 1.01 3.18 July 4.50 1.47 3.06 August 7.27 2.54 2.69 September 4.14 .00 2.50 October .83 3.11 November 1.85 4.21 December .00 4.80 'As shown in table I, considerably more rain fell in 1955 than in 1956 from January through September, being 34.57 inches for 1955 and 19.90 for 1956. The rainfall for 1955 for July, August, and September was 15.91 inches, almost twice the 44-average for the vicinity for these months; in addition, the rainfall for the first 9 months of 1955 was higher than the 44-year average for that period. October through December of 1955 added only 2.68 inches of water towards the moisture supply for 1956. A few total soil moisture determinations were made on June 20 at the edge of the branch spread of trees in a thinned check plot and in a thinned plot irrigated with 2 inches of water May 20 and 21. The data are given in table II. PROCEEDINGS TEXAS PECAN GROWERS ASSOCIATION 19 Table II. Total Soil Moisture, June 20, 1956, in Thinned Check and Thinned Plot Irrigated with 2 Inches of Water May 20 and 21. Depth Total Soil Moisture in________________ (Inches) Check Plot Plot Irrigated_______________ May 20 and 21 0-6 3.40 4.80 6-12 3.45 6.35 12-18 5.60 6.72 18-24 12.71 12.86 24-36 13.68 14.80 36-48 14.81 14.43 There was more moisture in the irrigated plot at all depths to 36 depths to 36 inches than in the check plots. At the 36-to-48 depth the percentage of moisture was slightly less in the irrigated plot. On July 16 and later considerable leaf drop from trees in the first six rows of the windward side of all plots took place. A drying south-west wind had been blowing for 3 days. The leaves of the trees were infected to a considerable extent by downy spot. The downy spot infection plus stress from lack of soil moisture apparently caused the leaf drop. As soon as irrigation water was applied July 18 and 19 the leaf droppings stopped in the irrigated plots but many trees had already lost about 25 percent of their leaves. One tree at the edge of the orchard was at a place where an irrigation pipe joint leaked considerably during the May 20 and 21 irrigation. This tree did not lose leaves in July when others near it lost part of theirs. Size of nuts was affected either by this water shortage in June and July or some other factor as large nuts running under 50 to the pound were not produced on any of the record trees. The average number of nuts to the pound, percentage of kernel, specific gravity of nuts, and volume of nuts are given in table III. Table III. Quality of Nuts from Matched Irrigated and Check Stuart Pecan Trees, 1956 Average Kind Water Nuts Volume Of Applied Tree per Specific of Plot_ acre-__ Spacing Pound Kernel Gravity Nuts___ Inches feet number percent cc Irrigated 11 60x80 70 46.52 .814 7.8 Irrigated 11 40x60 91 45.55 .806 6.3 Irrigated 9 60x80 71 46.29 .800 8.3 Irrigated 9 40x60 101 41.23 .714 6.2 Check 2 60x80 118 40.24 .754 5.3 Check 2 40x60 170 36.91 .687 3.8 As shown in table II the lowest average number of nuts to the pound, the highest percentage of kernel, and the highest specific gravity were for the thinned trees irrigated with 11 inches; the other irrigated thinned trees were a close second. The smallest and poorest quality nuts were from the check trees and the unthinned check trees produced the lightest and poorest nuts. The check thinned trees, however, produced nuts having a higher specific gravity than unthinned trees irrigated with 9 inches, indicating the importance of proper spacing so that trees do not crowd or compete with each other. All nuts except those from thinned irrigated trees were so small they were sold as seedlings at 21 cents per pound. The larger nuts sold for 25 cents per pound. One measure of the value of any treatment is the net return per acre. The manager of the orchard estimated his costs per acre as follows: irrigation water \$1.60 per acre-inch for electricity and labor in moving pipe; fertilizer\$8.00; seeding of cover crop and cultivation \$4.50; interest on cost of well and depreciation of irrigation equipment \$11.83, charged against the 11-inch irrigation plot and prorated to the 9-inch and 2-inch plots. A fair rental value of land was considered to be $6.00 an acre. Harvesting and handling costs of pecans were estimated at 5 cents per pound. The yields, gross return and net return per acre are given in table IV. Table IV. Yields and Gross and Net Returns per Acre from Irrigated and Check Trees, 1956. Kind Water Yield Price Of Applied Tree per per Return per Acre Plot___ acre- Spacing Acre Pound Gross Net___ inches feet pounds cents dollars dollars Irrigated 11 60x80 783 25 218 126 Irrigated 11 40x60 871 21 183 92 Irrigated 9 60x80 719 25 180 101 Irrigated 9 40x60 631 21 133 59 Check 2 60x80 398 21 84 40 Check 2 40x60 168 21 35 5 As shown in Table IV the highest yield, the largest gross return, and the largest net return per acre were from the thinned trees irrigated with 11 inches. The yield of the unthinned pecan trees irrigated with 11 inches was a close second but because of the lower price per pound for the smaller nuts the net return per acre was in third place. The yield per acre of the thinned trees irrigated with 9 inches was in third place, but these trees were second place in net return per acre. The net return from the thinned check (irrigated 2 inches) was approximately one-third of that of the thinned trees irrigated with 11 inches. The unthinned check (irrigated 2 inches) showed a net return of $5 per acre compared with $92 per acre for unthinned trees irrigated with 11 inches. DISCUSSION In this experiment best results were from thinned trees irrigated with 11 inches of water. One can only speculate what the results would have been if sufficient water had been given at the right time so that the growth of the nuts would not have been checked by a lack of water and no defoliation had taken place: undoubtedly yields and net returns per acre would have been increased on the best irrigated plots. The largest average size of nuts was 8.3 cubic centimeters volume per nut, about two-thirds the size of a large Stuart nut. Premature defoliation was definitely related to lack of water, for one of the most exposed trees did not defoliate where a leak in irrigation joint occurred May 20 and 21 and addition of water stopped defoliation. The Norfolk fine sand in the orchard does not have large water-storage capacity because of its inherent nature and comparative shallowness. Undecomposed rock is encountered by the soil auger at 7 to 8 feet. Because of this condition summer rainfall or supplemental irrigation in dry years is particularly important. The 44-year average rainfall for the vicinity shows an average total rainfall for July, August, and September of 8.25 inches. The 11-inch irrigation plots were given 7 inches of water during these three months and the total rainfall was 4.01, making a total of 11.01 inches. This amount was not sufficient to make large Stuart nuts. Thus, the average rainfall for July, August, and September is not adequate for best pecan development and production. The size of the crop on the trees is considered to have an effect on setting of next year’s crop, as a large crop is frequently followed by either a short crop or none at all. The trees in this experiment had no crop in 1955 because of freeze damage to the trees in March. Leaves free of diseases during August and September are considered to be a factor in setting of next year's crop. The trees held their leaves well in 1955. Soil moisture conditions are also considered a factor in setting of next year's crop. Good crops often follow seasons of fairly dry weather in August and September. August and September 1955 were far from dry and over twice the 44-year average rainfall of the vicinity fell in these months. An excellent crop of nuts occurred in this orchard in 1956. SUMMARY The thinned trees that received 11 inches of irrigation water had nuts of the largest size, highest percent kernel, and highest specific gravity. The unthinned check trees that received 2 inches of irrigation water had nuts of the smallest size, lowest percent kernel, and lowest specific gravity. Best yields and the highest net return were obtained from thinned trees receiving 11 inches of irrigation water. Second highest yields were from unthinned trees receiving 11 inches, but second highest net return was from thinned trees receiving 9 inches. The poorest yield and the poorest return were from unthinned check trees that received 2 inches of irrigation water in March. The irrigation water applied was not adequate to make large Stuart nuts.